In recent years, a GaN-based compound semiconductor material has attracted attention as a semiconductor material for short-wavelength light-emitting devices. The GaN-based compound semiconductor is formed on various oxide substrates such as sapphire single crystal or a Group III-V compound substrate by a metal organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy (MBE method) or the like.
A characteristic feature of the GaN-based compound semiconductor material is that the current diffusion in the transverse direction is small. This is attributable to many dislocations present in the epitaxial crystal and penetrating from the substrate to the surface, but the details are not known. Furthermore, in a p-type GaN-based compound semiconductor, the resistivity is high as compared with the resistivity of an n-type GaN-based compound semiconductor and, therefore, unless the material coming into ohmic contact with the p-type GaN-based compound semiconductor is made to be a positive electrode, the driving voltage greatly increases.
The material coming into ohmic contact with the p-type GaN-based compound semiconductor is mainly a metal and, in particular, a metal having a high work function readily establishes ohmic contact. Also, from the standpoint of light penetration, the positive electrode is preferably transparent to light. Therefore, a metal material which easily makes ohmic contact with a p-type GaN-based compound semiconductor has heretofore been formed into a thin film to thereby achieve both low resistance and light transparency.
When a metal is formed into a thin film, there arises a problem that the resistance in the diffusion direction becomes high. To solve this problem, a positive electrode having a two-layer structure consisting of an ohmic contact layer comprising a thin-film metal and a current diffusion layer comprising a transparent electrically conducting film having high light transmittance has been proposed (see, for example, Japanese Patent No. 294173).
In order to fabricate a brighter light-emitting device (LED), the presence of a metal layer which reflects or absorbs light must be eliminated. For this purpose, a method of bringing a transparent electrically conducting film itself into ohmic contact with a p-type GaN-based compound semiconductor has been studied (see, for example, Japanese Unexamined Patent Publication No. 2001-210867). In Japanese Unexamined Patent Publication No. 2001-210867, it is proposed to produce a transparent electrically conducting film coming into direct contact with a p-type GaN-based compound semiconductor by a method other than a sputtering method. In the sputtering method, the contact resistance is high because the p-type GaN-based compound semiconductor layer is damaged, and a low operating voltage cannot be obtained. However, when a transparent electrically conducting film is formed by a method other than sputtering and, then, the thickness of the transparent electrically conducting film is increased by a sputtering method, as the film-forming method is changed on the way of constructing a stacked structure, the resistance increases at the interface where the method is changed. Also, the productivity is poor.